Block compression, in which image data are compressed in units of blocks, is one of the most widely used techniques for compressing image data. A block referred herein is a unit of compression processing defined by dividing a frame image; a block is composed of a predetermined number of pixels arranged in a specific arrangement (for example, arrayed in n rows and m columns for n and m being integers at least one of which is two or more).
FIG. 1 schematically illustrates one example of definition of blocks. A frame image 100 is divided into blocks 101 arrayed in rows and columns. In the example illustrated in FIG. 1, each block 101 is composed of 2×2 pixels (pixels arrayed in two rows and two columns). The number of pixels included in each block 101 and the arrangement of pixels in each block 101 may be variously modified. For example, a block is defined as 8×8 pixels in the JPEG (Joint Photographic Experts Group) format as is well known in the art. The numbers of rows and columns of each block may be different from each other; for example, a block may be defined as 1×4 pixels (pixels arrayed in one row and four columns). Japanese Patent Application Publication No. 2010-11386 A discloses an example of block compression.
Block compression often includes a quantization process to reduce the number of bits. Most typically, a dither value determined for each pixel is used in a quantization process. In one example, a quantization process involves addition of a dither value to the grayscale value of each color or a value obtained from the grayscale value (for example, the luminance value), followed by bit truncation or rounding of the value obtained by the addition.
In general, it is preferable to suppress image deterioration in compression processing and this also applies to the block compression. As a result of inventors' study with respect to image deterioration in block compression including a quantization process, the inventors have discovered that a sort of image quality deterioration is caused by inappropriate generation of dither values used in the quantization process.